Haptic feedback in the form of vibration plays a role in the way users interact with their electronic devices. For example, vibration of mobile phones notifies a user of an incoming message or alarm. Motors that generate vibration in small form factor mobile devices tend to produce a limited range of vibration frequencies. Current motor technologies include small DC rotary motors that generate vibrations by spinning an unbalanced load and piezoelectric vibrators that typically vibrate near a single resonant frequency. As a result, mobile device designers are limited to using vibration patterns, such as ON-OFF and ON-OFF-ON-OFF, to differentiate different types of device notifications. Most humans are limited in their ability to reliably remember more than three to four different types of single-frequency ON-OFF vibration patterns and match them with a given type of notification.